Engines may be configured with an exhaust heat recovery system for recovering heat of exhaust gas generated at an internal combustion engine. The heat recovered at an exhaust gas heat exchanger may be utilized for functions such as heating the cylinder head, and warming the passenger cabin, thereby improving engine, and fuel efficiency. Cooled exhaust gas may be recirculated to the intake manifold and used to reduce fuel consumption, and exhaust NOx emissions. Further, EGR may be used to assist in the reduction of throttling losses at low loads, and to improve knock tolerance. An EGR cooler may be coupled to an EGR delivery system to bring down the temperature of recirculated exhaust gas before it is delivered to the intake manifold.
Various approaches are provided for exhaust heat recovery and EGR cooling. In one example, as shown in US 20140196454, Ulrey et al. discloses an engine system with a post-catalyst EGR cooler that may be opportunistically used to recover exhaust heat for heating the engine. During cold-start conditions, an exhaust throttle valve may be closed to direct exhaust through the EGR cooler wherein heat from the exhaust may be transferred to a coolant circulating through the EGR cooler. The coolant (warmed with the recovered exhaust heat) may then be circulated through the engine to increase engine temperature. During such cold-start conditions, the EGR valve may be maintained in a closed position, and after flowing through the EGR cooler, the exhaust may return to the main exhaust passage via a bypass passage.
However, the inventors herein have recognized potential disadvantages with the above approach. As one example, in the system shown by Ulrey et al., it may not be possible to restrict undesired exhaust flow though the EGR delivery passage and the bypass passage even when the exhaust throttle is in a fully open position. During higher than threshold engine temperature and engine load conditions, since it may not be possible to completely bypass the heat exchanger, undesired flow of hot exhaust through the coolant system may adversely affect the functionality of the coolant system. Also, it may not be possible to control concurrent exhaust heat recovery and cooled EGR delivery in order to improve fuel efficiency. Further, in the above mentioned heat exchange system, during use of the heat exchanger for EGR cooling, condensate may collect in the heat exchanger and can enter the intake manifold via the EGR passage, adversely affecting engine combustion stability. Condensate remaining in the heat exchanger during prolonged periods of engine shut down may freeze and can also cause damage to the EGR system components.
The inventors herein have identified an approach by which the issues described above may be at least partly addressed. One example engine method comprises: operating an engine exhaust system in a first mode with exhaust flowing to a tailpipe via a heat exchanger, and operating the system in a second mode with a first portion of exhaust recirculating to an intake manifold, and a second portion of the exhaust flowing to the tailpipe via the heat exchanger. In this way, EGR cooling and exhaust heat recovery may be simultaneously provided via a common heat exchanger.
In one example, an engine system may be configured with a heat exchanger positioned downstream of a catalytic convertor in an exhaust bypass disposed parallel to a main exhaust passage. A diverter valve may be used to enable exhaust gas to be diverted into the bypass passage, and through the heat exchanger. An EGR delivery passage may be coupled to the bypass passage downstream of the heat exchanger, and an EGR valve may be coupled to the delivery passage to control exhaust flow into the intake manifold. Adjustments to the position of the diverter valve and the EGR valve may be coordinated for exhaust heat recovery and EGR delivery. For example, during engine cold-start conditions, exhaust may be routed from the exhaust manifold to the tailpipe via the heat exchanger. During the flow, exhaust heat may be transferred to a coolant circulating around the heat exchanger, and the hot coolant may then be used for engine and cabin heating. After catalyst light-off, when cooled EGR is requested, the exhaust may be routed to the intake manifold via the EGR passage after flowing through the heat exchanger operating as an EGR cooler. Based on engine heating demand relative to EGR demand, a position of the diverter valve and the EGR valve may be adjusted to flow a first part of exhaust to the intake manifold via the heat exchanger and the EGR delivery passage, while simultaneously flowing a second (remaining) part of exhaust to the tailpipe via the heat exchanger. The controller may also adjust the ratio of the first part relative to the second part based on a comparison of fuel efficiencies in each mode. Further, a level of condensate formation at the heat exchanger may be estimated, and if the level of condensate is higher than a threshold level, the entire volume of exhaust may be routed to the tailpipe via the heat exchanger in order to purge the accumulated condensate to the atmosphere. The condensate may also be purged to the tailpipe responsive to an engine shut-down event.
In this way, by providing the functions of an EGR cooler and an exhaust gas heat exchanger via a single heat exchanger, cost and component reduction benefits are achieved without limiting the functionality or capability of either system. The technical effect of coupling the heat exchanger in an exhaust bypass passage connected to an EGR passage is that exhaust heat recovery and EGR flow can be provided simultaneously, increasing fuel economy benefits. By simultaneously providing EGR, and recovering exhaust heat for heating the engine and/or passenger cabin, fuel efficiency may be improved. The technical effect of opportunistically purging condensate accumulated in the heat exchanger to an exhaust tailpipe is that water ingestion in the engine may be reduced, improving combustion stability. Also, by purging the heat exchanger before an engine shut-down, freezing of water in the heat exchanger during cold periods may be reduced, thereby reducing the possibility of EGR component damage.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.